<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

<script src="https://cdn.bootcss.com/jquery/3.4.1/jquery.min.js"></script>

<form action="http://localhost:8080/client/login/auth" method="post">
    <input type="text" name="reqId" value="123">
    <input type="submit">
</form>

<script>

    function getQueryVariable(variable) {
        var query = window.location.search.substring(1);
        var vars = query.split("&");
        for (var i = 0; i < vars.length; i++) {
            var pair = vars[i].split("=");
            if (pair[0] == variable) {
                return pair[1];
            }
        }
        return (false);
    }

    $("input[name='reqId']").attr('value', getQueryVariable("reqId"))
</script>
</body>
</html>